The designators were dropped after the 2-1/2 year gap between the Challenger
incident (STS-51L), and the STS-26 "Return To Space". If you'd like an unprofessional
opinon about this system of mission designations, I'd persionally say it did what
it was designed to do. It got the public past any possible "STS-13" designation.
Comparisons with Apollo-13 would not have been appreciated.